Wastewater material may be settled out without treatment and is termed as primary sludge or may be treated by another process and is typically termed as secondary sludge. Collectively the term “sludge” is used for the material generated from wastewater treatment processes. The term “biosolids” is used for sludge that is further treated. Wastewater such as sewage streams generally contains various naturally occurring and/or man-made contaminants, notably organic contaminants. Some naturally occurring microorganisms have the ability to consume these contaminants for their own life processes, thereby turning an undesirable pollutant into (for their purposes) a beneficial nutrient or food source. The wastewater treatment industry frequently capitalizes on the ability of these microorganisms by using such microorganisms in facilities that treat wastewater streams to destroy the contaminants and break them down into basic compounds. Wastewater streams are fed into tanks and ponds that maintain conditions conducive to microorganism activity. Typically, the microorganisms that consume the target contaminants are mesophilic and thrive at temperatures in the range of about 15 to about 45 degrees Celsius.
The desired result of this type of wastewater treatment is the destruction of organic contaminants, but another result of this type of treatment is the production or increase of sludge, which includes these microorganisms. Although the sludge yielded from wastewater treatment operations can range from 0.1 pound of sludge per pound of biochemical oxygen demand (BOD) removed to about 1 pound of sludge per pound of BOD removed, a more typical range of sludge yield is from about 0.3 pounds to about 0.6 pounds of sludge per pound of BOD removed. Disposal of the sludge may still be problematic, even after many contaminants have been consumed by microorganisms. One problem arises from the human pathogenic nature of many microorganisms, such as the Fecal Coliform group of organisms. Although some microorganisms can be beneficial in consuming contaminants, they themselves pose a danger to human health and are disease-causing organisms. These include, but are not limited to, certain bacteria, protozoa, viruses and viable heiminth ova. Regulations by states, counties, cities, and/or the federal government often impose restrictions upon land disposal of material containing pathogenic microorganisms. It is desirable to treat sludge so that one can easily and legally dispose of the treated sludge now termed biosolids on land or under ground. Suitably treated biosolids may even prove to have beneficial uses. Under certain circumstances, biosolids may be used a soil conditioner or fertilizer similar to compost material.
Sludge also contains other materials including microorganisms which are not pathogenic in nature. Typically the sludge includes a group of microorganisms that thrive in what is generally referred to as the thermophilic temperature range, temperatures greater then 45 degrees Celsius. The thermophilic microorganisms are normally not harmful to humans, and there exists both aerobic and anaerobic bacteria that thrive within the thermophilic range. Thermophilic activity usually takes place within the range from about 45 degrees Celsius to about 75 degrees Celsius. In contrast, pathogenic bacteria usually thrive within a more narrow mesophilic range, which is from about 25 degrees Celsius to about 37 degrees Celsius, or the normal body temperature of humans. Some of these organisms may begin to die at about 38 degrees Celsius.
Various methods have been proposed and practiced for treating the sludge resulting from treatment of wastewaters. This sludge may be digested aerobically or anaerobically, with different microorganisms (biologically), as well as chemically, and/or physically. Sludge can also be thermophilically digested. Among the methods available for sludge treatment is autothermal thermophilic aerobic digestion (ATAD). ATAD capitalizes on the presence of materials in the sludge, such as naturally occurring microorganisms, which are not pathogenic or harmful to humans. Additionally, operations at these high temperatures inactivate the pathogenic microorganisms. Additional information regarding ATAD treatment of sludge is available in U.S. Pat. Nos. 5,948,261, 6,168,717, 6,203,701, 6,514,411 and 6,719,903 (assigned to Thermal Process Systems, LLC, of Crown Point, Ind.). An exemplary ATAD system is the ThermAer™ system available from Thermal Process Systems.
In a typical ATAD process, sludge resulting from wastewater treatment is aerobically and thermophilically digested in a reactor, which has sufficient oxygen available for aerobic microorganisms. The reactor operates at a temperature in the thermophilic range from about 45 degrees Celsius to about 75 degrees Celsius and even higher. Within this temperature range, thermophilic microorganisms are active, in an aerobic process where they utilize oxygen, for respiration, as they assimilate these contaminates.
The digestion of sludge, either aerobically or anaerobically, causes the mesophilic bacteria to break down, thereby generating certain byproducts when the protoplasm, from within the bacterial cells, is released into the biosolids. One of these byproducts is nitrogen which quickly converts to ammonia. Ammonia raises the pH of the digesting biosolids and may cause odors. Other byproducts include biopolymers, proteins, polysaccharides, and volatile fatty acids (VFAs). Higher concentrations of any of these byproducts often result in higher requirements of chemical conditioners and polymers for dewatering. It is thus desirable to have some way to reduce the amount of these byproducts.
To reduce these byproducts, various methods have been proposed and practiced for the conditioning of digested biosolids, including composting, anaerobic digestion, irradiation, pasteurization and drying. There are various concerns and disadvantages for each of these approaches for treating the byproducts, such as energy and chemical costs.
In addition, nitrification and denitrification steps have been used to treat wastewater, but their application to the treatment of digested biosolids has been limited. Existing nitrification and denitrification processes, for the post-digestion treatment of biosolids, have relied on the addition of large amounts of supplemental alkalinity, usually lime.
A major challenge in the conditioning of digested biosolids is that the temperature of the digested biosolids is typically from about 45 degrees Celsius to about 75 degrees Celsius, and therefore the digested biosolids must be cooled for conditioning. This is due to the mesophilic nature of the nitrifying and denitrifying microorganisms, which are optimized near but inhibited above approximately 40 degrees Celsius. Existing approaches for controlling and regulating the temperature of the conditioning tanks include the use of heat exchangers or natural convection.
There is a need for a nitrification/denitrification process for digested biosolids that is controllable, cost-efficient, and capable of alternating nitrification and denitrification phases in the same reactor. There is also a need for a nitrification/denitrification process for digested biosolids that does not rely on the addition of alkaline compounds such as lime.